Development of electrostatic images



United States Patent 3,010,842 DEVELOPMENT OF ELECTROSTATIC IMAGESEugene C. Richer, Columbus, Ohio, assiguor, by mesne assignments, toXerox Corporation, a corporation of New York N0 Drawing. Filed Aug. 29,1955, Ser. No. 531,280 1 Claim. (Cl. 117-37) This invention relates to amethod for the development of electrostatic latent images.

In xerography it is usual to form :an electrostatic latent image on asurface. One method of doing this is to charge a photoconductiveinsulating surface and then dissipate the charge selectively by exposureto a pattern of activating radiation. Other means of formingelectrostatic latent images are set forth in U.S. 2,647,464 to James P.Ebert. Whether formed by these means or any other, the resultingelectrostatic charge pattern is conventionally utilized by thedeposition of an electrostatic material thereon through electrostaticattraction whereby there is formed a visible image of electroscopicparticles corresponding to the electrostatic latent image.Alternatively, the electrostatic charge pattern may be transferred to aninsulating film and the electroscopic particles deposited thereon toform the visible image. In any case, this visible image, in turn, may betransferred to a second surface to form a xerographic print.

The process of depositing the electroscopic powder on the electrostaticimage to render the electrostatic image visible is called thedevelopment step and is one of the most critical steps of the entireprocess. As the electrostatic latent image is developed by deposition ofan electroscopic material thereon, it is obvious that the process offersfreedom from dependence on the awkward and complicated liquiddevelopment processes of conventional photography. To emphasise thisfreedom the process has been named xerography as generally dry powderswere used as the electroscopic materials. However, this is notnecessarily the case as is shown in U.S. 2,551,582 to C. F. Carlsonwherein the electroscopic material was applied in the form of a liquidspray.

It is evident that no picture can be better than the development steppermits. About the coarsest type of image reproduced by a xerographicprocess requires a resolution of at least about 50 lines per inch,Commercial linecopying xerographic machines generally have a resolutionpower of about 125 to 250 lines per inch. The process used in obtainingthis resolution is termed cascade development and is set forth in US.2,618,552. This process involves the use of a finely-divided coloredmaterial colored called a toner deposited on a slightly morecoarsely-divided material called a carrier. component developer iscascaded across the electrostatic image areas.

The control of the concentration of toner in the twocomponent developerbecomes difficult on continued operation of a xerographic machine. Whenapplied to continuous-tone development where resolutions greater thanabout 300 lines per inch are necessary it has been found impossible toobtain the desired quality of reproduction using. such a system.

xerographic development known as powder cloud development has beenevolved. In this process a powder cloud is generated by agitating a massof dry electroscopic material in a confined space or in the case of aliquid by spraying through an atomizer. The cloud so produced is chargedand then contacted with the electrostatic latent image. This processmakes possible the realization of extremely high resolutions suchas 1200or more lines per inch. Furthermore, concentration of toner is moresusceptible to control so that the process is easily adapt- I able foruse on continuous machines.

This two- Accordingly, another system of 3,010,842 Patented Nov. 28,1961 However, the process also has its defects. Thus, to realize thehigh resolutions possible with this system and to obtain solid-areacoverage it is necessary to use a development electrode-that is aconductive electrode spaced close enough to the image-bearing surface todraw the lines of force externally above the image-bearing surface. Dueto the low dielectric constant of air an extremely close spacing of thedevelopment electrode is desired for its effective use. This introducesa number of difficulties due to the extreme difficulty of obtainingadequate air flow in the very narrow and confined space between thedevelopment electrode and the image-bearing surface. Other difiicultie-sarise in attempting to obtain a constant flow for extended periods oftime from the powder cloud generator and in flowinga uniform powdercloud over all areas of the image bearing surface. Moreover powderclouds are not uniformly charged to the same polaritya significantportion of the cloud will be of the opposite polarity to that desiredfor the development of the image. These oppositely charged particles inthe cloud tend to deposit from the powder cloud on uncharged areas ofthe'image making difficult the obtaining of a clear white on such areas.Finally, the equipment forgenerating, charging and directing the flowingof the powder cloud becomes rather complicated.

The instant invention relates to a process which represents asubstantial improvement in the art of xerographic development. In thisprocess the electroscopic material is suspended in a liquid rather thana gas and the electrostatic latent image developed by contacting theimagebearing surface with the liquid suspension to produce a layer ofliquid on the image-bearing surface whereby the electroscopic materialis attracted out of the liquid to the image-bearing surface as byimmersion of the image-bearing member in the suspension, by flowing thesuspension over the image bearing surface or otherwise. This process hasseveral advantages over the prior art: Thus, development is almostinstantaneous. Contacting the liquid suspension of powder particles withthe electrostatic image to form a layer of the liquid suspension thereonis easily and quickly accomplished without the necessity for complicatedand cumbersome equipment. The particles are believed to be all chargedwith the same polarity (probably a zeta-potential) without the necessityfor special charging equipment. The use of a liquid permits a higherconcentration of useable developer particles than when gas is thesuspending medium. The image defects due to irregularities in the airflow are obviated and, finally, the use of a liquid having a highdielectric constant permits more effective use of a developmentelectrode.

The process of the invention will now be set forth in more detail. Y

According to the present invention a dispersion of electroscopicparticles is formed in a suitable liquid for example as by stirring orotherwise as is well known to those skilled in the art. Suitableelectroscopic particles include any pigment or similar material having afine particle size and being insoluble in the suspending liquid. Ingeneral the finer the powder the better the grain in the developedimage. It is preferred that the average particle I quality work it ispreferred that the average particle size burnt sienna, ultramarine blue,Hausa yellow, rose bengale, Dupont Luxol blue, and methyl cellulose. Asalready mentioned the pigment should not be soluble in the liquid and itmust also be so selectedrelative to the liquid used as to assume acharge of the desired polarity. The

electrostatic charge imparted to suspended particles is a function bothof the nature of the particles and of the suspending liquid.

The liquid used must have a relatively high insulating value to avoiddischarge of the latent electrostatic image. Suitable liquids includehydrocarbons such as benzene, xylene, hexane, naphtha, kerosene, etc.;halogenated hydrocarbons such as carbon tetrachloride,trichloroethylene, chloroform, Genetrons (a trade name of Allied ChemicaCo. for various fluorinated compounds), Freons (a trade name of E. I. duPont de Nemours & Co. for various fluorinated compounds), etc.;miscellaneous liquids such as turpentine, etc.

Any method known to those skilled in the art may be used for dispersingthe pigment in the liquid. In general simple stirring or agitation ofthe liquid is sufficient. Where a highly uniform and stable suspensionis desired, the suspension formed by stirring may be passed through acolloid mill, but excellent suspensions may be attained without suchtreatment. where they do not impair the insulating properties of theliquid.

The density of pigment deposited on the xerographic plate is dependenton the charge of the electrostatic latent image, the pigmentconcentration in the liquid and the presence of a development electrode.As is the case in powder cloud development described above thedevelopment electrode consists of a conductive electrode, either ofcontinuous conductive material as sheet metal or of a conductiveforaminous grid, closely spaced to the i-magebearing surface. Thefunction of the development electrode is to draw the lines of force ofthe electrostatic image externally above the image-bearing surface. Thisgives improved solid-area coverage and, by increasing the amount of thelines of force that are external to the imagebearing surface and henceavailable'to attract electrostatically charged marking particles,improves the overall efiiciency of the development process. Where theliquid suspension is contacted with the electrostatic image-bearingsurface in a conductive dish or tank, the walls of the container willthemselves constitute a development electrode. Moreover, the use ofliquids having much higher dielectric constants than air permits theeffective use of a development electrode at considerably greaterspacings from the image-bearing surface than when air is the interveningdielectric.

The developed images are satisfactory for visual inspection on the platewhile still wet. If desired, the wet image may be transferred byabsorption by paper. If desired, the image may be dried and transferredto a desired base such as paper, plastic, etc. by the usual methods ofelectrostatic transfer as described in US. 2,576,047 to R. M. Schaffertor adhesive transfer as described in US.

2,661,289 to Mayo and Wolfe. The image, in turn, may 1 then be fixed byany means known to those skilled in the art such as those described, forexample, in US. 2,297,691 to C. F. Carlson. Thus where either thepigment itself fusible by heat or solvent vapor, or where the sheet'towhich it is transferred is coated or impregnated with a material fusibleby heat or solvent vapor, then the use of either heat or solvent vaporwill permanently aifix the image to the sheet. Alternatively, anothermethod of fixing the image comprises spraying the surface of the sheetbearing the image with a fixative lacquer by atomizer. Other means oftransferring and/or fixing the image known to those skilled in the artmay be used.

.In the following examples a xerographic plate comprising a micron layerof selenium on an aluminum backing was charged and exposed to a patternof light andshadow in'a XeroX Copier and XeroX Camera Unit Suspendingagents may be used No. 1 commercially available from the Haloid Companyv of Rochester, New York. In each case the electrostatic 4 Example IExamples 217 Liquid suspensions were prepared by dispersing 0.5 g.

of each of the following materials in cc. of naphtha and stirring with asteel rod. Development was as above.

Material Comments Sulphur No continent.

Poor image. Charcoal D Good reversal development.

Good positive development.

Exceptionally good positive image.

No comment.

Apparent reversal development.

Apparent reversal development.

Sharp and bright letters.

Positive image. Good solid area coverage.

Poor image.

Positive image. Good solid area coverage. Quite sharp.

No comment.

Reversal development.

Poor contrast and definition.

Too much deposition.

Fairly good continuous tone development.

Dupont Luxol Blue Rose Bengals Powdered Brass Cobalt blua. Yellow ochreAmerican Vermilion.

Burnt umber Lithopone Burnt Sienna Cadmium red Chrome yellow (med)Powdered aluminum Methyl cellulose Example 18 A suspension was preparedby stirring 0.5 g. finelydivided charcoal into 100 cc. steam-distilledturpentine. An image was developed as in Example 1. Deposition waslight.

Example 19 A liquid suspension was prepared by placing in a glasscontainer 1.0' g. of finely-divided charcoal and 100 cc. of carbontetrachloride. The container was then stoppered and dispersion completedby manually shaking a few times.

The suspension was then poured into a metal tray and the image-bearingsurface contacted with the suspension by immersing the xerographic platetherein in the dark. The plate used in this example consisted of a 50micron layer of selenium on a brass backing. After immersion the platewas withdrawn from the suspension, the lights turned on and the imagetransferred while wet by contact with dye-transfer paper. By thisprocess a. completely developed xerographic image was obtained in just afew seconds and with only the simplest of equipment.

Examples 20 and 21 Suspensions were prepared as in Evample 1 using 0.1g. finely-divided aluminum and 0.1 g. finely-divided copper, in eachcase dispersed in 100 cc. of naptha. Development was in Example 1. Bothproduced good images, the aluminum having slightly better resolution.

Example 22 Example 23 A suspension was prepared by stirring 4; teaspoonof an alcohol dag obtained from Acheson Colloids, Port Huron, Michigaminto cc. of naphtha. The image was developed as in Example I. The imagewas thin due to light deposition.

Examples 24 and 25 Suspensions were prepared as in Example 1 using 0.5g. of charcoal and 0.25 g. of aluminum powder, in each case dispersed in100 cc. of xylene. The images were developed as in Example 1. Fairimages were obtained.

Example 26 A suspension was prepared by stirring both 0.1 g. of charcoaland 0.1 g. of aluminum powder into 100 cc. of naphtha. The image wasdeveloped as in Example 1. The developed image was a mixture of both thecharcoal and the aluminum.

The increased versatility of xerography due to the availability of suchalternative development processes is obvious at once. Thus it ispossible to develop an image either positively or negatively byselecting a liquid-pigment system wherein the charge on the particlesis, respectively, either opposite to or the same as the polarity of thecharge of the electrostatic image. Furthermore, it is possible toproduce images of mixed colors by blending pigments of different colorbut similar charge polarity. It is also possible to simultaneouslydevelop both the image and the background by using two pigments whichtake on the opposite polarity of charge when dispersed in the sameliquid. While a. particular embodiment of the invention has beendescribed, it is not intended to be limited thereto, but it is intendedto cover the invention broadly within the spirit and scope of theappended claim.

I claim:

A process of developing an electrostatic latent image on an insulatingsurface which comprises contacting an electrically insulating layerbearing an electrostatic latent image with an electrically insulatingliquid so that all of the image-bearing portions of said layer contactsaid liquid, said liquid having dispersed therein finely-dividedparticles of a first color and finely-divided particles of a secondcolor, the particles of said first color and the particles of saidsecond color having opposite polarities as a result of being dispersedin said liquid, and simultaneously developing both the image andbackground on said insulating layer.

References (Zited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Metcalfe: Liquid Developers for Xerography, Journal ofScientific Instruments, vol. 32, February 1955, pp. 74 and 75.

Atkinson et al.: Ferrography, Journal of the Franklin Institute, vol.252, No. 5, November 1951 pp. 373-381.

